In 1990 Robert Trotter was identified as the creator of 52 forged paintings—one is shown on the left in the image. Experts in the art industry could better recognize fakes like Trotter’s if they knew the precise ages of the canvas and the paint. Traditional radiocarbon dating methods could do the job, but the milligram-mass samples needed would mar a potentially one-of-a-kind piece. Now Laura Hendriks from ETH Zürich and her colleagues have capitalized on advancements in accelerator mass spectrometry and gas ion sources to date much smaller samples.
In the new approach, researchers first clean and then combust a solid sample to produce carbon dioxide, which is then carried into the mass spectrometer’s gas ion source. There the sample is converted to a stable stream of negative carbon ions that are then accelerated to high speed, allowing the carbon-12 and carbon-14 isotopes to separate from one another. The method only requires a few hundred micrograms of material: The acceleration step improves the abundance sensitivity and eliminates other interferences.
Hendriks and her team tested the dating method on Trotter’s known fake. The inset on the right in the image shows where the authors’ colleague acquired a sample (blue triangle) of white paint and canvas fiber: It’s close to a spot (dashed blue lines) where paint had already begun to chip off. The team used a roughly 200 µg sample to analyze the age of the paint layers and the canvas fiber. The oil that bound the pigments contained excessive 14C, which suggested to the authors that the seeds from which the oil was extracted were harvested after nuclear testing began in the early 1950s. (The neutrons produced by nuclear bombs can collide with atmospheric 14N to eventually make 14C.) The wide age range of the canvas, made sometime between the late 1600s and the mid 1900s, stems from the highly variable 14C levels in the atmosphere before 1950. The age difference between paint and canvas reveals Trotter’s mode of deception: He used modern paint on old canvas.
The new method isn’t foolproof. Multiple layers of varnish or other additional conservation efforts could smear an otherwise definite age estimate. At the very least, the new radiocarbon technique provides an additional, complementary tool for analysts to identify faux paintings. (L. Hendriks et al., Proc. Natl. Acad. Sci. USA, 2019, doi:10.1073/pnas.1901540116.)
